Method and system for image rejection by using post mixer I/Q equalization

ABSTRACT

The invention provides a system and method for tuning broadband signals by using post mixer I/Q equalization. An Image Rejection Mixer (IRM) is used for mixing Radio Frequency (RF) signals and rejecting image signals from the desired RF signals. The IRM includes an I/Q mixer and a filter. The I and Q paths resulting from the mixing operation in the I/Q mixer are equalized in amplitude and phase by an I/Q equalizer. Thereafter, the image signals are rejected from the desired RF signals using the filter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to broadband tuners. Morespecifically, it relates to systems and methods for image rejection fromdesired Radio Frequency (RF) signals.

2. Description of the Related Art

Broadband tuners are used in various electronic devices, such astelevisions (TV), for transmission and reception of RF signals atparticular frequencies. Broadband tuners include RF mixers that are usedto convert RF signals at a particular frequency to RF signals at anotherfrequency. This conversion is usually referred to as mixing RF signals.Mixing RF signals involves mixing of Local Oscillator (LO) signals withthe input broadband signals. The output RF signals of the RF mixerusually include the desired RF and image signals. The image signals areunwanted signals that need to be rejected from the output RF signals, toprovide the desired signals at the output. Filters such as SAW filters,variable bandpass filters, and polyphase filters are used to reject theimage signals from the output RF signals.

Broadband tuners usually provide up to 40 dB of image rejection.Broadband TV tuners require up to 70 dB of image rejection for bettertransmission of the desired RF signals. In the present state of thetechnology, there are methods that provide up to 70 dB of imagerejection. These methods require the implementation of double-conversiontuners with fixed IF SAW filters, single-conversion tuners with complexinput tracking filters or single-conversion tuners with wide bandmatching techniques before the mixing operation. One such singleconversion tuner with a double quadrature mixer is described in apublication titled “CMOS Mixers and Polyphase Filters for Large ImageRejection”, by Farbod Behbahani, Yoji Kishigami, John Leete and Asad A.Abidi, and published in IEEE Journal of Solid-State Circuits, June 2001,vol. 36, No. 6.

However, in the above methods and systems, the size of the circuitand/or its non-linearity is increased. Moreover, the wide band matchingtechnique reduces the signal-to-noise ratio of the broadband tuners.

In light of the above, there is a need for a system and method forbroadband tuning with the desired amount of image rejection. Further,the system should provide high linearity and high signal-to-noise ratiowithout substantially increasing the size of the circuit.

SUMMARY OF THE INVENTION

An object of the invention is to provide a desired amount of imagerejection in RF transmission, independent of mixing operation.

Another object of the invention is to provide the image rejectionwithout increasing the size of a tuner.

Another object of the invention is to the provide image rejection withhigh linearity in RF transmission.

Yet another object of the invention is to provide a high signal-to-noiseratio in RF transmission.

To achieve the above-mentioned objectives, the invention provides asystem and method for image rejection in RF transmission. Imagerejection greater than 70 dB is achieved if the input signals to thefilter are matched. The system utilizes a post mixing equalizationtechnique for equalizing the input signals provided to the filter. Theamplitudes of the differential output signals of an I/Q mixer areequalized and their phases are aligned in quadrature. The difference inamplitudes of the differential output signals is sensed by a leveldetect circuit and the amplitudes are equalized by adjusting the gainsof Variable Gain Amplifiers (VGA). Further, phases of the differentialoutput signals are aligned in quadrature by addition of these signals.Polyphase filters, for the transmission of the desired RF signals,thereafter reject the image signals

In accordance with the invention, various circuit blocks of the systemcan be utilized in the analog as well as the digital domain to providethe desired amount of image rejection. Further, the circuit blocks maybe designed by using various types of transistor devices and topologies.Further, the size of the circuit is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference tovarious embodiments, some of which are illustrated in the appendeddrawings. It is to be noted, however, that the appended drawingsillustrate only typical embodiments of this invention and are thereforenot to be considered limiting of its scope, for the invention may admitto other equally effective embodiments.

FIG. 1 is a view of a block diagram depicting an exemplary prior arttuner in which various embodiments of the invention may be practised;

FIG. 2 is a view of a block diagram of an image rejection mixer (IRM),in accordance with various embodiments of the invention;

FIG. 3 is a view of a schematic representation of various elements ofthe IRM, in accordance with an embodiment of the invention;

FIG. 4 is a view of a vector diagram depicting a phase alignment ofdifferential signals of I and Q paths, in accordance with variousembodiments of the invention; and

FIG. 5 is a view of a flowchart depicting a method for tuning broadbandsignals, in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

Various embodiments of the invention provide systems and methods fortuning broadband signals. Broadband signals are tuned by rejecting imagesignals from the desired RF signals by using a post mixer equalizationtechnique. During the equalization, the differential output signals ofan I/Q mixer are equalized in amplitude. Further, the phases of thedifferential output signals are aligned in quadrature by the addition ofthese signals. Filtering the desired RF signals provides the desiredamount of image rejection.

FIG. 1 is view of a block diagram depicting an exemplary prior art tuner100 in which various embodiments of the invention may be practised.Tuner 100 includes an Image Rejection Mixer (IRM) 102 and a demodulator104. IRM 102 includes a mixer 106 and a filter 108. Tuner 100 tunes theinput RF signals to desired RF signals. In one embodiment of theinvention, tuner 100 is a broadband tuner.

Mixer 106 converts the input RF signals at a particular frequency to RFsignals at another frequency. During the mixing operation, two signals,i.e., the RF signal and an image signal are obtained at the output ofmixer 106. Filter 108 rejects the image signal, i.e., the unwantedsignal, and passes the desired RF signals. The desired RF signals areobtained as Intermediate Frequency (IF) signals at the output filter108. Demodulator 104 demodulates the IF signals to obtain the desiredaudio/video signals.

In various embodiments of the invention, the input RF signals arebroadband signals. In various embodiments of the invention, the inputbroadband RF signals are supplied through a Community Access Television(CATV) infrastructure. In various embodiments of the invention, mixer106 may be an I/Q mixer and filter 108 may be a polyphase filter.

FIG. 2 is a view of a block diagram of IRM 102, in accordance with anembodiment of the invention. IRM 102 includes an I/Q mixer 202, an I/Qequalizer 204, and an IF polyphase filter 206. I/Q equalizer 204includes one or more amplitude equalizers 208, hereinafter referred toas amplitude equalizer 208, and a phase aligner 210.

I/Q mixer 202 converts input broadband RF signals into an in-phase (I)and a quadrature phase (Q) path. IRM 102 provides the desired amount ofimage rejection, i.e., more than 70 dB of image rejection when the I andQ output paths of I/Q mixer 202 have equal amplitudes and phases inexact quadrature.

Amplitude equalizer 208 equalizes the amplitudes of the I and Q paths.The I and Q paths, after amplitude equalization, are provided to phasealigner 210. Phase aligner 210 aligns the phases of the I and Q pathswith equalized amplitudes in quadrature. The phases are aligned byadding the I and Q paths. In various embodiments of the invention, theamplitudes of the I and Q paths may not be equal after the phases of theI and Q paths are aligned in quadrature. Hence, they need to beequalized again. Amplitude equalizer 208 equalizes the amplitudes ofphase-aligned the I and Q paths.

The amplitude equalized and phase-aligned signals are provided to IFfilter 206. IF polyphase filter 206 passes the desired RF signal fromthe I and Q paths and rejects the image signals.

FIG. 3 is a view of a schematic representation of various elements ofIRM 102, in accordance with an embodiment of the invention. IRM 102includes I/Q mixer 202, one or more channel reduction filters 306,amplitude equalizer 208, phase aligner 210, and IF polyphase filter 206.I/Q mixer 202 includes an I mixer 302, a Q mixer 304 and a LocalOscillator 308. Amplitude equalizer 208 also includes one or moreVariable Gain Amplifiers (VGA) 310 and a level detect circuit 312. Phasealigner 210 includes four adders 314, hereinafter referred to as adders314.

Differential input broadband RF signals are provided at the input portsof I mixer 302 and Q mixer 304. In various embodiments of the invention,the frequency of broadband RF signals may be in a range of 57 MHz to1100 MHz. Local Oscillator 308 generates differential quadratureoscillator signals. I mixer 302 and Q mixer 304 mix the input broadbandRF signals with the differential quadrature oscillator signals. Theoutput paths of I mixer 302 and Q mixer 304, i.e., the I and Q paths,are also differential. In various embodiments of the invention, the Iand Q paths may have unequal amplitudes and phase mismatch due toprocess variations of I mixer 302 and Q mixer 304. The I and Q paths ofI/Q mixer 202 are thereafter provided to channel reduction filters 306.Channel reduction filters 306 eliminates LO leakage out of I/Q mixer 202and reduces the number of broadband signals from the I and Q paths. Inone embodiment of the invention, LO leakage from I/Q mixer 202 is low,and hence, channel reduction filters 306 may not be required. In variousembodiments of the invention, the LO leakage from I/Q mixer 202 must beless than 16 dB lower than the minimum RF output from I/Q mixer 202.

Amplitude equalizer 208 equalizes the I and Q paths by means of VGAs 310and level detect circuit 312. VGAs 310 amplify the differential signalsof the I and Q paths. The amplified output signals of VGAs 310 areprovided to level detect circuit 312. Level detect circuit 312 detectsthe difference in the amplitudes of the I and Q paths and accordinglyadjusts the gains of VGAs 310.

Adders 314 add the differential signals of the I and Q paths withequalized amplitudes. An exact phase match of the differential signalsof the I and Q paths is obtained by addition. The addition of thedifferential signals of the I and Q paths results in four differentialoutput signals, hereinafter referred to as the I′ and Q′ paths. The I′and Q′ paths include the following signals: I₀+Q₀, I₀+Q₁₈₀, I₁₈₀ +Q₀ andI₁₈₀+Q₁₈₀. These four differential I′ and Q′ signals are in exactquadrature phase with each other. (Phase alignment by means of additionis described in detail in conjunction with FIG. 4.)

In various embodiments of the invention, the amplitudes of thedifferential signals of the I′ and Q′ paths may not be equal. In thisevent, the differential signals of the I′ and Q′ paths are provided toamplitude equalizer 208. Amplitude equalizer 208 equalizes theamplitudes of the I′ and Q′ paths. The output signals of amplitudeequalizer 208 are provided to IF polyphase filter 206. IF polyphasefilter 206 rejects the image signal and passes the desired RF signal asIF output signals. In various embodiments of the invention, thefrequency of the RF signals present in the IF output signals may vary ina range of 0 to 4 MHZ.

In various embodiments of the invention, channel reduction filters 306are low pass filters. VGAs 310 may be for example, transistoramplifiers, operational amplifiers and the like. In one embodiment ofthe invention, the various circuit elements of IRM 102 may be designedusing by Complementary Metal Oxide Semiconductor (CMOS) technology. Inanother embodiment of the invention, the various circuit elements of IRM102 may be designed by using Bipolar Junction Transistors (BJT).

FIG. 4 is a view of a vector diagram depicting the phase alignment ofthe differential signals of the I and Q paths, in accordance withvarious embodiments of the invention. This is explained with the help oftwo signals, resulting from the addition of the differential signals ofthe I and Q paths, i.e., I₀+Q₀ and I₁₈₀+Q₀. Signals I₀+Q₀ and I₁₈₀+Q₀are hereinafter denoted by the X and Y signals, respectively. The phasedifference φ_(diff) between the X and Y signals is calculated with thehelp of equation (1):

$\begin{matrix}{\Phi_{diff} = {\frac{\pi}{2} - {\cos^{- 1}\left( \frac{2 \cdot {A} \cdot {B} \cdot {\sin(\theta)}}{\sqrt{{4 \cdot {A}^{2} \cdot {B}^{2} \cdot {\sin^{2}(\theta)}} + \left( {{A}^{2} - {B}^{2}} \right)}} \right)}}} & (1)\end{matrix}$

Further, the difference in the amplitudes of the X and Y signals isdetermined by the following equation (2):

$\begin{matrix}{{{X} - {Y}} = {\sqrt{\left( {{{B} \cdot {\cos(\theta)}} + {A}} \right)^{2} + {{B}^{2}{\sin^{2}(\theta)}}} - \sqrt{\left( {{{B} \cdot {\cos(\theta)}} - {A}} \right)^{2} + {{B}^{2}{\sin^{2}(\theta)}}}}} & (2)\end{matrix}$

In the equations given above, A and B are the amplitudes of thedifferential signals of the I and Q paths, and θ is the phase differencebetween them. The phase difference, φ_(diff), is equal to 90 degrees forall values of θ, when the magnitude of A and B are equal, i.e., theamplitudes of the differential signals of the I and Q paths are equal.The difference in amplitude between X and Y signals is nonzero for θ notequal to 90 degrees.

Amplitude equalizer 208 equalizes the amplitudes of the X and Y signals.In various embodiments of the invention, the output RF signals, i.e.,the output signals of amplitude equalizer 208 form the I′ and Q′ paths.The differential signals of the I′ and Q′ paths are equal in magnitudeand are in exact quadrature phase. This provides more than 70 dB ofimage rejection for transmission of the desired RF signals.

FIG. 5 is a view of a flowchart depicting a method for tuning broadbandsignals, in accordance with an embodiment of the invention. At step 502,an I/Q mixer mixes the input broadband RF signals. During the mixingoperation, the RF signals at a particular frequency are converted intoRF signals at another frequency. The I/Q mixer generates I and Q paths.At step 504, the difference in the amplitudes of the differentialsignals of the I and Q paths is sensed. Thereafter, the amplitudes areequalized by amplifying the signals based on the difference.

At step 506, the phases of the I and Q paths with equalized amplitudesare aligned in quadrature. They are aligned in quadrature by addingdifferential signals of the I and Q paths. The addition of thedifferential signals of the I and Q paths (I₀, I₁₈₀, Q₀ and Q₁₈₀),results in differential I′ and Q′ paths (I₀+Q₀, I₀+Q₁₈₀, I₁₈₀+Q₀ andI₁₈₀+Q₁₈₀) that are in exact phase quadrature. In various embodiments ofthe invention, the amplitudes of the differential signals of the I′ andQ′ paths, obtained after phase alignment, are unequal. At step 508, theamplitudes of the differential signals of the I′ and Q′ paths areequalized, to generate the desired output RF signal. At step 510, theimage signals are filtered out from the output RF signals. The filterrejects the image signals and passes the desired RF signal. This resultsin an IF signal at the output port of the filter. In various embodimentsof the invention, the resulting output IF signal is in the range of 4MHz.

In one embodiment of the invention, the system and method as describedin conjunction with FIG. 3 and FIG. 5 may be utilized in a broadbanddigital data system. In an alternative embodiment of the invention, thesystem and method as described in conjunction with FIG. 3 and FIG. 5 maybe utilized in a broadband analog system.

The system and method described above has a number of advantages. Thesystem provides an image rejection of more than 70 dB withoutsubstantially increasing the size of the tuner. Further, the systemprovides high linearity and high signal-to-noise ratio in RFtransmission. The analog circuits of the TV tuner, as described in thepresent invention, do not require any particular transistor device type,and can be integrated by using CMOS technology. The system for TV tuningcan also be implemented in the digital domain. Further, the system forequalizing the I and Q outputs of an I/Q mixer can be used to equalizeany arbitrary I/Q signal to improve the I and Q amplitude and phasematch.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A broadband tuner comprising: an I/Q mixer, the I/Q mixer mixinginput broadband signals; at least one amplitude equalizer, the at leastone amplitude equalizer equalizing amplitudes of I and Q paths generatedby the I/Q mixer; a phase aligner, the phase aligner aligning phases ofthe I and Q paths with equalized amplitudes to generate an output RFsignal, wherein the phases are aligned in quadrature; and a polyphasefilter, the polyphase filter filtering an image signal from the outputRF signal.
 2. The broadband tuner according to claim 1, wherein the atleast one amplitude equalizer comprises a plurality of Variable Gainamplifiers (VGA), the plurality of VARs amplifying the I and Q paths. 3.The broadband tuner according to claim 1, wherein the at least oneamplitude equalizer comprises a level detect circuit, the level detectcircuit sensing difference in amplitudes of the I and Q paths.
 4. Thebroadband tuner according to claim 1, wherein the phase alignercomprises at least four adders, the at least four adders addingdifferential signals of the I and Q paths.
 5. The broadband tuneraccording to claim 1, wherein at least one of the I/Q mixer, the atleast one amplitude equalizer, the phase aligner and the polyphasefilter are designed using analog design techniques.
 6. The broadbandtuner according to claim 1, wherein at least one of the I/Q mixer, theat least one amplitude equalizer, the phase aligner and the polyphasefilter are designed using digital design techniques.
 7. The broadbandtuner according to claim 1, wherein at least one of the I/Q mixer, theat least one amplitude equalizer, the phase aligner and the polyphasefilter are designed using complementary metal oxide semiconductor (CMOS)technology.
 8. The broadband tuner according to claim 1, wherein theinput broadband signals are supplied through a Community AccessTelevision (CATV) infrastructure.
 9. A broadband digital data systemutilizing the broadband tuner of claim
 1. 10. A broadband analog systemutilizing the broadband tuner of claim
 1. 11. A method for tuningbroadband signals, the method comprising the steps of: mixing inputbroadband signals, wherein the mixing is performed by an I/Q mixer;equalizing amplitudes of I and Q paths generated by the I/Q mixer;aligning phases of the I and Q paths with equalized amplitudes togenerate phase-aligned I and Q paths, wherein the phases are aligned inquadrature; equalizing amplitudes of the phase-aligned I and Q paths togenerate an output RF signal; and filtering an image signal from theoutput RF signal.
 12. The method according to claim 11, wherein the stepof equalizing amplitudes of I and Q paths comprises the step of sensingdifference in amplitude of the I and Q paths.
 13. The method accordingto claim 11, wherein the step of aligning phases of the I and Q pathscomprises the step of adding differential signals of the I and Q pathswith equalized amplitudes.
 14. The method according to claim 11, whereinthe input broadband signals are supplied through a Community AccessTelevision (CATV) infrastructure.